#define _CRT_SECURE_NO_WARNINGS 1
struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2) {
    struct ListNode* tail = NULL, * head = NULL;
    if (l1 == NULL)
        return l2;
    if (l2 == NULL)
        return l1;
    if (l1->val < l2->val)
    {
        head = tail = l1;
        l1 = l1->next;
    }
    else {
        head = tail = l2;
        l2 = l2->next;
    }
    while (l1 && l2)
    {
        if (l1->val < l2->val)
        {
            tail->next = l1;
            tail = l1;
            l1 = l1->next;
        }
        else
        {
            tail->next = l2;
            tail = l2;
            l2 = l2->next;
        }
    }
    if (l1 == NULL)
    {
        tail->next = l2;
    }
    if (l2 == NULL)
    {
        tail->next = l1;
    }
    return head;
}